I spent an hour at a tramp park in Utah (Snogression) only working on spins on the tramps. After 30 minutes I went from struggling with corking my 360s to being able to do really smooth controlled 720's (a guy was coaching me a bit). When I got back on the hill I was able to throw down some 360's I hadnt been able to do. So yeah it helps. Im too much of a wuss to try anything bigger then a 360 though :/
oh so one of the exercises I did (this is for slyder) do a FS 180 then when you land do an immediate BS 180 once you land do a FS 360, then BS 360, keep adding a 180 rotation once you complete a set. so 180, 180, 360, 360, 540, 540. If you mess up start back at the 180. It worked really well for me.